Growth factor treated quiescent cells respond by expressing immediate early genes (IEGs) that do not need new protein synthesis for their expression. Many of these genes such as c-fos whose transcription is activated by growth factors are known oncogenes. c-fos has a sequence element in its promoter termed the Serum Response Element (SRE) which binds Serum Response Factor (SRF). Myocardin and Megakaryoblastic Leukemia-1 (MKL1) are members of a family of transcriptional co-activators that bind to SRF and activate transcription from promoters with SRF- binding sites. MKL1 is also involved as the chromosome 22-encoded protein altered by the translocation in acute megakaryoblastic leukemia of infants and young children. The translocation fuses the genes for MKL1 and RNA-Binding Motif protein 15 (RBM15);the resulting RBM15-MKL1 fusion protein is believed to possess oncogenic properties. MKL1 is found either in the cytoplasm or the nucleus;it has RPEL, Basic, Glutamine-rich, SAP, Leucine zipper-like and transcriptional activation domains. The RPEL motifs bind actin and are essential for serum- regulated control of nuclear localization of MKL1 in some cell types. The basic region is required for SRF binding and mediates the nuclear localization of MKL1. The leucine zipper region mediates homo- and hetero-dimerization of the MKL1 family members. SAP region's function is not well understood but has been involved in nuclear matrix attachment in other proteins. MKL1 is required for the RhoA-dependent SRE activation and MKL1 overexpression strongly activates SREs. It is unclear how RhoA regulates MKL1. Cellular localization of MKL1 was reported to be regulated by its association with actin, but our lab has found that MKL1 is constitutively nuclear in some cell types and that nuclear localization of MKL1 is not sufficient to activate SREs. In order to understand how MKL1 is regulated I will identify proteins that interact with MKL1 and determine their role in its regulation by doing the following: 1) Identify MKL1-interacting proteins. 2) Map the site(s) of interaction on MKL1. 3) Study how these interactions regulate early gene transcription. . It is hoped that these experiments will identify additional growth regulatory signaling steps in the pathway from RhoA to MKL1. In addition, we will better understand how oncogenes such as c-fos are regulated. If the mechanism of action of deregulated genes in cancer is understood we may be able to elucidate new therapeutic strategies.